Fix SDL::Window#fullscreen

[wooster0]

Right now it gives you this when you use it:

Error in wool.cr:46: instantiating 'Wool::Window:Module#fullscreen(Bool)'

  Wool::Window.fullscreen true
               ^~~~~~~~~~

in window.cr:29: instantiating 'SDL::Window#fullscreen=(Bool)'

   window.fullscreen = true
          ^~~~~~~~~~

in lib/sdl/src/window.cr:86: undefined constant LibSDL::WindowFlags::WINDOW

      self.fullscreen = value ? LibSDL::WindowFlags::FULLSCREEN : LibSDL::WindowFlags::WINDOW
                                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~

[ysbaddaden]

Thanks!